Water operator math tutorial
Pounds Formula (lb/day)
Convert a flow and a concentration in mg/L into pounds per day.
The pounds formula is the workhorse of operator math. It turns a flow in million gallons per day and a concentration in milligrams per liter into the weight of chemical or solids moved each day.
Chemical feed, solids loading, and BOD loading all start here, so most other formulas on this page cluster are variations of it.
The formula
lb/day = flow (MGD) × concentration (mg/L) × 8.34 lb/gal
| Variable | Unit |
|---|---|
| Flow | MGD |
| Concentration | mg/L |
| Result: Load | lb/day |
Conversion factors used: 8.34 lb per gallon of water.
Applies to: Water Treatment, Water Distribution, Wastewater Treatment.
Common exam tip: Write MGD and mg/L next to each number before you multiply. If a flow is in gpm, convert to MGD first.
Calculator
Load
62.55 lb/day
- 1. 2.5 MGD × 3 mg/L × 8.34 = 62.55 lb/day
For instructional use. Results assume the standard conversion factors shown on this page. Actual field calculations may need chemical strength, density, temperature, and other adjustments, so check your plant’s procedures.
Worked examples
Example 1
A plant treats 2.5 MGD and feeds a chemical at a 3 mg/L dose.
- 2.5 MGD × 3 mg/L × 8.34 = 62.55 lb/day
Answer: 62.55 lb/day
Example 2
A 0.75 MGD system targets a 12 mg/L concentration.
- 0.75 MGD × 12 mg/L × 8.34 = 75.06 lb/day
Answer: 75.06 lb/day
Common mistakes
Wrong result: 7.5 lb/day
leaving out the 8.34 conversion factor
Wrong result: 0.9 lb/day
dividing by 8.34 instead of multiplying
Practice questions
1. Flow is 4.2 MGD at 1.8 mg/L. How many pounds per day?
- A63.05 lb/day
- B7.56 lb/day
- C0.91 lb/day
- D630.5 lb/day
Show answer and explanation
Correct answer: A. 4.2 MGD × 1.8 mg/L × 8.34 = 63.05 lb/day Answer: 63.05 lb/day. A common slip is leaving out the 8.34 conversion factor, which gives 7.56 lb/day.
2. Flow is 1.4 MGD at 25 mg/L. How many pounds per day?
- A35 lb/day
- B291.9 lb/day
- C4.2 lb/day
- D2,919 lb/day
Show answer and explanation
Correct answer: B. 1.4 MGD × 25 mg/L × 8.34 = 291.9 lb/day Answer: 291.9 lb/day. A common slip is leaving out the 8.34 conversion factor, which gives 35 lb/day.
3. Flow is 0.6 MGD at 8 mg/L. How many pounds per day?
- A4.8 lb/day
- B0.58 lb/day
- C40.03 lb/day
- D400.3 lb/day
Show answer and explanation
Correct answer: C. 0.6 MGD × 8 mg/L × 8.34 = 40.03 lb/day Answer: 40.03 lb/day. A common slip is leaving out the 8.34 conversion factor, which gives 4.8 lb/day.
Where this formula applies
This calculation can appear in the disciplines below. Exact exam coverage depends on your state, certification level, and official exam outline.
Water Treatment
Water Distribution
Wastewater Treatment
Study guides
Keep going with the full guide
Water Operator Math Made Simple walks through operator math step by step and includes printable references plus a full practice exam.
Water Operator Math Made Simple
A Step-by-Step Guide to the Math Every Water Operator Needs to Know